Power/Performance Bits: Oct. 22

Thermal emitter improves solar cell efficiency
Stanford University scientists have created a heat-resistant thermal emitter — an element used in specialized solar cells — that could significantly improve the efficiency of the cells. The heat-resistant thermal emitter is designed to convert heat from the sun into infrared light that can be absorbed by solar cells to make electricity – a technology known as thermophotovoltaics.

Earlier prototypes fell apart before temperatures reached 2,200 degrees Fahrenheit but the new thermal emitter remains stable at temperatures as high as 2,500 F, marking a record performance in terms of thermal stability and a major advance for the field of thermophotovoltaics, the researchers said, which included participants from the University of Illinois-Urbana Champaign and North Carolina State University.

A typical solar cell has a silicon semiconductor that absorbs sunlight directly and converts it into electrical energy but silicon semiconductors only respond to infrared light. Higher-energy light waves, including most of the visible light spectrum, are wasted as heat, while lower-energy waves simply pass through the solar panel. This means in theory, conventional single-junction solar cells can only achieve an efficiency level of about 34%, but in practice they don’t achieve that because they throw away the majority of the sun’s energy. However, thermophotovoltaic devices are designed to overcome that limitation. Instead of sending sunlight directly to the solar cell, thermophotovoltaic systems have an intermediate component that consists of two parts: an absorber that heats up when exposed to sunlight, and an emitter that converts the heat to infrared light, which is then beamed to the solar cell.

The light is tailored to shorter wavelengths that are ideal for driving a solar cell, which raises the theoretical efficiency of the cell to 80%. So far, thermophotovoltaic systems have only achieved an efficiency level of about 8% largely due to problems with the intermediate component, which is typically made of tungsten – an abundant material also used in conventional light bulbs. These thermal emitters developed by Stanford and the others have a complex, 3D nanostructure that has to withstand temperatures above 1,800 F [1000 C] to be practical, and in fact, the hotter the better.

The cross-section micrograph of the thermal emitter shows the ceramic-coated tungsten retained structural integrity after being subjected to 2,500 F (1,400 C) for one hour. (Source: Stanford University)

The results demonstrated for the first time that ceramics could help advance thermophotovoltaics as well other areas of research, including energy harvesting from waste heat, high-temperature catalysis and electrochemical energy storage. The researchers plan to test other ceramic-type materials and determine if the experimental thermal emitters can deliver infrared light to a working solar cell.

Lego-like structures improve solar cell efficiency
In other solar cell research news, scientists from Imperial College London and international collaborators in Belgium, China and Japan have demonstrated that the efficiency of all solar panel designs could be improved by up to 22% by covering their surface with aluminium studs that bend and trap light inside the absorbing layer. At the microscopic level, the studs make the surface of the solar panels look similar to Legos.

A solar panel with rows of aluminium studs and large electrical connections. The studs have been enlarged here but would normally be so small that they are invisible to the naked eye.(Source: Imperial College London)

While in recent years both the efficiency and cost of commercial solar panels have improved, they remain expensive compared to fossil fuels. As the absorbing material alone can make up half the cost of a solar panel the researchers’ aim has been to reduce to a minimum the amount that is needed. The success of this technology, in combination with modern anti-reflection coatings, will go a long way down the path towards highly efficient and thin solar cells that could be available at a competitive price, the researchers believe.

In their work, the researchers attached rows of aluminium cylinders 100nm across to the top of the solar panel, where they interact with passing light, causing individual light rays to change course. More energy is extracted from the light as the rays become effectively trapped inside the solar panel and travel for longer distances through its absorbing layer. In the past scientists have tried to achieve the light bending effect using silver and gold studs because those materials are known to strongly interact with light, however these precious metals actually reduce the efficiency as they absorb some of the light before it enters the solar panel.

They said the key to understanding these new results is in the way the internal structures of these metals interact with light. Gold and silver both have a strong effect on passing light rays, which can penetrate into the tiny studs and be absorbed, whereas aluminium has a different interaction and merely bends and scatters light as it travels past them into the solar cells. An additional advantage to this solution is that aluminium is cheaper and far more abundant than silver and gold.